Melting ice sheets alarm scientists

World at a Glance

Regions in Greenland, Antarctica shrinking faster at their edges

New satellite information shows that ice sheets in Greenland and western Antarctica continue to shrink faster than scientists thought and in some places are already in runaway melt mode.

British scientists for the first time calculated changes in the height of the vulnerable but massive ice sheets and found them especially worse at their edges. That's where warmer water eats away from below. In some parts of Antarctica, ice sheets have been losing 30 feet a year in thickness since 2003, according to a paper published online Thursday in the journal Nature.

Some of the areas are about a mile thick, so they still have plenty of ice to burn through. In parts of Antarctica, the yearly rate of thinning from 2003 to 2007 is 50 percent higher than it was from 1995 to 2003.

The new measurements, based on 50 million laser readings from a NASA satellite, confirm what some of the more pessimistic scientists thought: The melting is in a self-feeding loop. The more the ice melts, the more water surrounds and eats away at the remaining ice.

The study does not answer the question of how much this will add to projections of sea level rise from man-made global warming. Some scientists have estimated that steady melting of the two ice sheets will add about 3 feet to sea levels by 2100. But the ice sheets are so big it would probably take hundreds of years for them to disappear.